Add coverage

[inpefess]

Closes #516
Breaks unit-tests support for Cygwin.
Coverage data collection takes time, but if one can run the tests in parallel, it takes under one minute (tInterpretedFunctions.cpp is the longest-running).

[MichaelRawson]

I'm generally happy, thanks for the fiddly work reviving coverage!

@joe-hauns @RobCoutel - I believe you are the main unit testers. Is this useful for you, and do you mind the increased timeout? If it's a problem we could think about how to make the tests run faster or putting coverage behind a flag.

[joe-hauns]

I didn't ever use any code coverage analysis for c++ but I think any additional linters are great to have so I'm very happy about adding this (without having it tried out yet)!

[inpefess]

It seems that master doesn't build with tests enabled now on Cygwin anyway: #519

My branch builds on Cygwin (not in Debug mode) to no surprise since I've changed _exit only in unit-test related code.

For gcc/clang (@MichaelRawson asked earlier) --- they both have --coverage flag, I'm testing locally wih gcc, and CI seems to use clang.